Frequency-stabilization of oscillators



July l0, 1951 L, E, NORTON v 2,560,365

FREQUENCY STABILIZATION OF OSCILLATORS ATTORNEY Juy 10, 1951 l... E.NORTON 2,560,365

' FREQUENCY STABILIZATION oF osoILLAToRs lNvENToR -AZOWeBY/E. orionATTORNEY July l0, 1951 L. E. NORTON FREQUENCY STABILIZATION OFOSCILLATORS 3 Sheets-Sheetl 5 Filed oct. 22. 1949 Patented July l0,l1951 FRE QUENCY- S TABILIZATIN F OSCILLATRS Lowell E. Norton, Princeton,N. J., assigner to Radio Corporation of America, a corporation ofDelaware App-lisation october 22, 1949, serai No. 122,938

(ci. o-36) 21 Claims. l

This invention relates to methods of and systems for stabilization ofthe frequency of oscillators, particularly micro-wave oscillatorslsuchas klystrons, magnetrons and thelike. I

In accordance with the invention, upper and lower sidebands produced byvmodulation of the oscillator to be stabilized are beat against acyclically' varying lo'w frequency to' produce two sweeping sidebandswhich pass in' opposite directions thr'ough the standard frequency ineach sweep cycle. The difference, in the sweep cycle, between the timesat which the standard frequency is swept by the two sidebands israprcise measure of the frequency deviation. of the oscillator and isutilizable for control of the oscillator frequency.

More specifically,v the sweeping sidebands are impressed upon high-Qcircuit devices or elements resonant at-the standard frequency whicheither may be the desired operating. frequency of the oscillator or afrequency oifset therefrom to prechosen extent. Voltage envelopes of thefrequency swept high-Q: devices are demodulated for impressionupontheinput circuits of a phasecomparator or coincidence detector whoseoutput voltage is utilized or utilizable for controliof the oscillatorfrequency. For stabilization of microwave oscillators, thel high-Qdevices comprise'conflned bodies of gas exhibiting sharp molecularresonance at a desired standard frequency.

The invention further' residesy in methods and systems having novel anduseful features. hereinafter described and claimed.Y

For a incre detailed understanding ofA the invention, reference is madeto the 'accompanying drawings in which:

Figure 1 is a block diagram of a frequency-stabilizing system;

Figures 2A-2C and Figures 3 andY 3C are eX- planatory figures referredto in discussionof Fig. 1 and other gures Y Figures 4 and 5 are blockdiagrams of modifications o'f the system ofv Figure l in which thestandard frequency is oifset' from theV desired z operating frequency ofthe oscillator;

Figurev 6 is an explanatory gure referred to in discussion of Figures 4and 5;

Figure 7 is a block diagram of still another modication of theinvention.

Similar reference characters are applied to similar elements throughoutthe drawings.

Referring to Figure l, at least a portion of the output of theoscillator I0 to be stabilized is impressed upon a mixer I2y upon whichthere is also impressed the low-frequency output of anoscillator orother low frequency source I3, thus to produce upper and lower sidebandsFU and FL respectively. When the oscillator Ill supplies power to anantenna or other load generically represented by block 9, an attenuator24 should be interposed between mixer I2 and the loadto avoid undesiredmodulation` ofV that portiony of the oscillator output supplied to theload and not used as: hereinafter described forY frequency-stabilizationpurposes. The output circuit ofthe mixer I2V is divided intotwo paths,respectively including lters MA and MB, which selectively transmit thesidebands. Specically,.the filter I4A may pass the upper sideband FU tothe-eX- clusion of thezlower sideband FL, and conversely, the filter MBpasses the lower sideband FL to the exclusion. of the upper sideband FU.The outputs of the filters MA, IllB,f. or equivalent4 frequencyselectivecircuits or devices, are respectively impressed upon the mixers IEA andIEB upon which is impressed a relatively low frequency Fis which iscyclically varied by a mechanical or electrical modulator of anysuitable type generically represented by sweep frequency generator I'I.vPreferably, and for'reasons which later appear, the cyclically-va'ryinglow-frequency energies supplied to the mixersV I5A and I5B are derivedfrom a common source I6.A

The output of the mixer I5A therefore includes an upper sweepingsideband periodically passing through a range of frequencies higher thanthe` upper sideband frequency FU and also a lower sweeping sideband'covering a range ofv frequencies-lower than the' upper sidebandfrequencyFU. The output of the other'mixer IEB alsovincludes upper andlower sidebands sweepingranges of frequency respectively higher andlower than the lower sideband frequency FL. The oper-ating frequency FMof oscillator I3 and the range of frequencies swept by oscillator IB areso chosen that in each sweep cycle of the oscillator I6, the uppersweeping sideband FLH of frequency FL and the lower sweeping side bandFui. of the higher frequency FU pass through a standard frequency Fswhich in this' form of the invention corresponds with the desiredoperating frequency of oscillator IU. v

The sweeping sideband outputs of the mixers IEA and HSB are respectivelyimpressed upon high-Q circuit devices H3A and IB, each sharply .resonantat the standard frequenpy Fs so that "the output of each. of the high-Qelements IBA- from discussion of Figures 2A-2C and Figures 3-3C, uponand is a direct measure of the difference between the operatingfrequency F of oscillator I and the standard frequency Fs, the resonantfrequency of the high-Q elements IHA, I8B.

The pairs of pulses so derived from the sweeping sidebands in successivesweep cycles are impressed upon a phase-detector or phase-comparator I9which for automatic stabilization of the frequency of oscillator Il! isof any known type producing a unidirectional output voltage varying insense and magnitude in accordance with variation between the phaserelation of the pulses applied to its input circuits. This outputvoltage may be applied, as by line 20, to effect stabilization of thefrequency of oscillator I0 in any of various known manners, some ofwhich are later herein specifically referred to.

Although the invention is not limited thereto, it is particularly suitedfor stabilization of microwave oscillators, in which event the standardfrequency is most desirably a molecular resonance frequency of a gas. Asmore fully set forth in copending applications including Serial Nos.5603 and 8246, many gases, including ammonia, exhibit sharp molecularresonance at one or more fixed frequencies which are not affected byambient conditions including temperature.

For utilization of a molecularly resonant gas as a standard offrequency, a body thereof at suitably low pressure is confined in anenclosure such as a resonant cavity or section of waveguide so to form ahigh-Q circuit element utilizable as the standards ISA, I8B of Figure 1.As shown in the aforesaid applications, the gas chamber may have sealedwindows of mica or like material transparent to microwave energy foreffective inclusion of the combined bodies of gas as sharply resonantcircuit elements respectively between mixer IA and demodulator 2I A inone channel of the servo-system and between mixer I5B and demodulatorIIB in the other channel of the servo-system.

To give a specic example, helpful in a better understanding of theinvention, it is assumed that the desired operating frequency F of theoscillator I0 is 23,870.1 megacycles which corresponds with the 3, 3line of ammonia. In such case each of the high-Q elements ISA and I8B isa chamber for confining ammonia at suitably low pressure, for example,0.01 millimeter of mercury so to afford sharp molecular resonance of thegas frequency standard at 23,870.1 megacycles (Fs). Further by way ofexample, the operating frequency of the oscillator IS may be megacycles,in which case the upper sideband frequency FU is 23,885.1 megacycles andthe lower sideband frequency FL is 23,855.1 megacycles. Further by wayof specific example, it will be assumed that the frequency of oscillatorI6 sweeps a range of from 10 to 20 megacycles, and that the sweepwaveform as shown in Figure 3 is sawtoothed. Thus, in each sweep cycle,as shown in Figure 3A, the upper sweep FLH of frequency Fr. increasesfrom 23,865.1 to 23,875.1 megacycles, while at the same time the lowersweeping sideband For. of frequency FU decreases from 23,875.1 to23,865.1 megacycles. Thus, in each sweep cycle of oscillator I6, the twosweeping sidebands pass through the standard frequency Fs from oppositedirections and under the condition of zero frequency-deviation ofoscillator Iii Figures 2A and 3A, there is time coincidence of suchpassages of the sweeping sidebands through the standard frequency Fs of23,870.1 megacycles.

In the foregoing sweep operation, two other sidebands also occur. Thelower sweep FLL of frequency F1. decreases from 23,845.1 to 23,835.1megacycles while at the Fu increases from 23,895.1 to 23,905.1megacycles. However, since neither of these sweep intervals passesthrough the standard frequency Fs, there is no steep voltage-frequencycharacteristic and neither of these two sidebands contributes anythingto the output or operation.

When, however, as shown in Figure 2B, the operating frequency F ofoscillator I0 has drifted to the value higher than the standardfrequency Fs, the upper sweeping sideband Fui of the frequency FL passesthrough the standard frequency Fs ahead of the lower sweeping sidebandof frequency FU in each sweep cycle. This is graphically illustrated inFigure 3B.

When, on the other hand, the frequency F oscillator I0 drifts to a valuelower than the standard frequency Fs, as shown in Figure 2C, the lowersweeping sideband of frequency FU passes through the standard frequencyin each sweep cycle before the upper sweeping sideband of frequency Fr.does: this is graphically illustrated in Figure 3C.

From the foregoing discussion of Figures 2A to 2C and 3-3C, it should beunderstood that the phase or time relation between the pulses producedby demodulation of the outputs of the high-Q devices IBA and I8B variesin sense and magnitude with the frequency-deviations of oscillator I0from the desired operating value. Such shift in phase relation can beobserved by applying the pulses to an oscilloscope whose sweep frequencycorresponds with the sweep frequency of oscillator I6. Using anelectronic switch, or equivalent, to apply the pulses in rapidalternation to the tube, the coincidence or lack of coincidence of thepairs of pulses is evident. This type of phase-comparator can be usedfor manual control of the frequency of oscillator I0, the operator atmore or less frequent intervals, or continuously, manually adjusting thefrequency control of oscillator I0 to maintain coincidence of the pulsepairs on the oscilloscope screen. Preferably, however, thephase-comparator is of a type such as shown in copending applicationsSerial Nos. 4497 or 115,698 which produces a control voltage applicableto the klystron or magnetron oscillator tube, or a control tubetherefor, continuously automatically to maintain the pulses of each pairin time coincidence. The sweep frequency or repetition rate of theoscillator I6 should be sufficiently high for example, kc. if the Q ofFs does not exceed 105, for collection of the frequency-errorinformation at time intervals short compared to drift of the frequencyof oscillator I0 and also should be high compared to any modulation ofthe frequency of oscillator I0 at intelligence-conveying frequencies.

When the desired operating frequency F of microwave oscillator IB doesnot correspond with the molecular resonant frequency of a gas but issomewhat offset therefrom, there may be utilized a system such as shownin Figure 4 in which the operating frequency of the oscillator I0 ismodulated more than once to obtain the sidebands which sweep thestandard frequency Fs. Assuming, as indicated in Figure 6, the standardfrequency Fs is higher than the desired operating frequency F ofoscillator I0, the output of oscillator I0 may be modulated byoscillator 23 to produce in the output circuit of mixer 22 upper andlower sidebands F2, F1, (Figure 6) and the upper assunse.

sideband-frequency `F2 .is modulated by. oscillator I3;4 as-inthe systemof `FigurelI to produce sideband frequencies Fr.v and Fir-respectivelylower and-higher than the standard frequency Fs.

From this pointonpthevsysterrrof Figure: 4 and.

the operation thereof is the` same as that of Figure lv andfurt-herdescription is unnecessary.

Shouldvinthesystem of- Figurel', it `beldesired: to Hmodulatextheoperating frequency F- of oscillator lll for'transmissionofv audio orvideo sig nals-by.= a frequency-modulated carrier,thiszintelligence-convey-ing modulationmay be `introducedv into eitherYor both 'of Vthe input circuits-` of phase-detector ISbeyOndfthefilters-14A and MB.

Specifically, the output' offfmodulator 26 mayfbe applied" to vary thephase of` the output pulses of--one or bothof the-higheQ devices 18A,IBB at speech orvideo frequencies; For more complete/description of theprinciples of operation of such a-modulating arrangement andfordisclosure of circuits suited therefor, reference is madeI -tooopendingapplications Serial Nos. 135;'780 and1'15,698. This same methodof modulation may-:be usedjinthe system of-Figure 1 when oscillator IUisvto beused for communication or` thuslproduced, as shown infFigure 6;upperandv lower sidebandfrequencies F1, F2,- one ofwhich is modulated,as by oscillator I3.,v to produce -the 'up-Y per and lower sweepingsidebands For., Fun-which pass through the 1. standard.` frequency= FsAin each sweep-cycle. Fromithispointon, the circuit-'componentsofafFigure 5, andtheir operation is the same; as .discussed in`connection with `Figure v 1.

fIrf it isdesiredV tomodulate the operatingfre-y quency off the;oscillator-for transmission of intelligence, this maybe effectedy asdiscussed in.

COIlIlfGL L I1.Wi-Jflf Figure l by. introducingthe modulation signal`into either one or. bothv .of the control; channels; `beyond the filtersMA.- I AB.

Infthesystem showngin Figure 7, the .upper and lowersidebandsFnv and;which are modun lated,` by oscillator -IE` to; produce the.- sweepingsidebands,A are. produced by modulating the poe tential of ainom-linear; control; electrode ofthe;

oscillator tube or of anassociated control tube by-modulator 32. As;in-the system, of Figure l, forf'example, thegsideband frequenciesFU-andFL are; selectivelytransmitted-bythe filters., l 4A, and |4B^tothe-mixersl5 A.-|5B of ;the,twovchannels offthe-servo-system. Despitetheffactthatsin this case the-upperY andl ylower -sidebands' FL, Fu arenot `of xed'- frequency, they; nevertheless are always equallyv spacedaty everyinstant from the standard frequency Fs. Consequently, thelpairsof pulses produced by demodulation of, the out.

puts of standards: l BA, l 8B.l are coincident inthe successive sweepcycles only when there is -coin-y cidence between the mean carrierfrequency of oscilla-tor Ill and thestandard frequency Fs, the resonantfrequencyfof" the high-Q circuit `devices iB lA, andflB When-thereisdeviation-.from thel desired operating-frequency,gthe.shift inpli-eisere-A lation'-A of thepulsesapplied -to the input circuits,`A

ofdetectorv I9 causes the output voltagethereof correspondingly` to varyand so `returnthev oscil1- lator frequency to the desiredvalue.,

-In' this, as in all of the other systems herein4 disclosed, the methodandsystem is particularly suited forstabilization rofthe frequency`ofimicrowave oscillators with respectto the ,molecular resonantvfrequency..y ofk al, gas.v For microwave operation,rthe` mixers ,anddemodulators are pref,-

erablyof the crystal rectifier type,Y the circuit connectionsl ll,Y lIA, I IB between the microwave components ,of the system `are waveguidesori-con. centric lines andv lters 14A, IABi and attenuator 2,4 are` ofknowntypes suited for operation-at microwave frequencies.

' From the foregoing discussion and speci'c'examples, it shall be'Junderstood the invention is not limited tothe particulararrangementsshown and thatchanges and modifications maybemade within theSCODQV `0f the,- appended claims.

What isy `claimed is:

li, The method of stabilizing'the operating free.

quency kof an oscillator withrespect to a standard frequency whichcomprisesmodulating oscilla'- tions generated;` bysaid oscillator toproduce cyclically varying sidebands A respectively having meanifrequencies higher and lower than said standard frequency whi-chinasweep cycleyof'V said` sideloands,` pass through said standard freFquency irroppositel sense,k producing control sig,

nals as. said sweeping sidebands pass` through. saidstandardfrequency,and controlling the-,free quency of saidoscillator inlrespronse to,saidcontrol; signals .to maintain aA lixedtime-- relation ber. tweensuccessive control signals :in predetermined sweep -cycles of saidvarying;beatfrequency.

2.,'Themethod of stabilizingthe operatingffre quency of' anoscillator'with respect to astandamdv frequency ywhich comprisesmodulating the genl erated oscillationsat least oncel to producesidebands; respectively; higher andylowerl lthan-saidV standardfrequency, mixing saidv upper vand-lower sidebands with. ay cyclicallyvarying beat fre-` quencyV to produce two sweepingsidebands which inasweep-cycle of: said varying beat frequency passfthrough said: standardfrequency in ,opposite directions, producing a pair vof, pulses in. eachsweep cycle, each pulsey occuringas one of said sweeping sidebandspasses through said standard' frequency, and controlling the frequencyvofsaidv` oscillator closely to maintain a fixed timexrelation,betweenthe pulses'of successive pairs: thereofinsuccessive sweepicyclesof said varying beat frequency.

3.i The method offstabilizing the operating frequency ofl an oscillator.at a standard frequency which comprises modulating; the generatedoscil-V lationsz to produce sidebandfrequencies equally spaced aboveandbelow the oscillator frequency, mixing said sideband'; frequencies-.withacycli'- callyly varying-beat frequency to produce twosweepingsidebands; whichin a; sweepcycle 4of said varying beatfrequency` respectively pass through said@ standardifrequency fromoppositev directions producing'in each of said cycles'a. pair of pulseseachoccurring asfone-of saidsweeping sidebands passes through`saidstandard free quency, and controlling the frequency of'saidoscillator closely` to -maintain coincidence of thev pairsv of pulsesinsuccessive sweep cycles of said' varying beat frequency.

. 4'.: The method. of"A stabilizing an-oscillatorvQ-at anl operating.frequencyfoifset .from a .standard frequency which comprises modulatingthe generated oscillations to produce upper and lower sidebands,modulating one of said sidebands to produce a second pair of sidebandsof frequencies respectively higher and lower than said standardfrequency, mixing said second sideband frequencies with a cyclicallyvarying beat frequency to produce two sweeping sidebands which in asweep cycle of said varying beat frequencies respectively pass throughsaid standard frequency from opposite directions, producing in each ofsaid cycles a pair of pulses each occurring as one of said sweepingsidebands passes through said standard frequency, and controlling thefrequency of said oscillator to maintain coincidence of the pairs ofpulses in successive sweep cycles of said Varying beat frequency.

5. The method of stabilizing an oscillator at an operating frequencyoffset from `a standard frequency 'which comprises modulating the inputof the oscillator to produce upper and lower sidebands, modulating oneof said sidebands to produce a second pair of sidebands whosefrequencies are respectively higher and lower than said standardfrequency, mixing said second sideband frequencies with a cyclicallyvarying beat frequency to produce two sweeping sidebands `which in asweep cycle of said varying beat frequency respectively pass throughsaid standard frequency from opposite directions, producing in each ofsaid -cycles a pair of pulses each occurring as one of said sweepingsidebands passes through said standard frequency, and controlling thefrequency of said oscillator closely to maintain coincidence of thepairs of pulses in successive sweep cycles of said varying beatfrequency.

6. The method of stabilizing at a standard frequency the mean-carrierfrequency of an oscillator which is modulated for transmission ofintelligence which comprises mixing upper and lower sidebands producedby aforesaid modulation with a beat frequency which cyclically varies atsupermodulation frequency to produce two sweeping sidebands which in asweep cycle of said beat frequency respectively pass through saidstandard frequency from opposite directions, producing in each of saidcycles a pair of pulses each occurring as one of said sweeping sidebandspasses through said standard frequency, and controlling the frequency ofsaid oscillator closely to maintain coincidence of the pairs of pulsesin successive sweep cycles of said varying beat frequency.

7. The method of stabilizing the operating frequency of an oscillatorwith respect to a standard frequency which comprises modulating thegenerated oscillations at least once to produce sidebands respectivelyhigher and lower than said standard frequency, mixing said upper andlower sidebands with a cyclically varying beat frequency to produce twosweeping sidebands which in a sweep cycle of said varying beat frequencypass through said standard frequency in opposite directions, producing apair of pulses in each of said cycles, each pulse occurring as one ofsaid sweeping sidebands passes through said standard frequency, derivingfrom said pulses a unidirectional voltage varying in sense and magnitudewith variations of the time relation between the pulses of the pairs insuccessive sweep cycles, and applying said voltage to said oscillator tovary the frequency thereof in sense minimizing variation of theaforesaid time relation of the pulses.

8. The method of stabilizing the operating frequency of a microwaveoscillator with respect to 8 a molecular resonance frequency or a gaswhich comprises modulating said oscillator to produce side-bandfrequencies respectively higher and lower than said molecular resonancefrequency, mixing said upper and lower sidebands with lowfrequencyoscillations to produce a second pair of sidebands respectively higherand lower than said molecular resonance frequency, cyclically varyingsaid low frequency to effect sweeping by said second pair of sidebandsof ranges of frequency including said molecular resonance frequency,respectively impressing said sweeping sidebands upon confined bodies ofsaid gas, demodulating the sideband energies transmitted by said bodiesof gas to produce pairs of pulses occurring as said sweeping sidebandspass through said molecular resonance frequency of said gas, andcontrolling the-frequency of said oscillator closely to maintaincoincidence of the pulses of each ,i

pair in the successive sweep cycles of said low frequency.

9. The method of stabilizing the operating frequency of a microwaveoscillator system at the molecular resonancefrequency of a gas whichcomprises modulating said oscillator to produce sidebands respectivelyhigher and lower than said molecular resonance frequency throughout therange of drift of said operating frequency, mixing said upper and lowersidebands with a cyclically varying low frequency to produce sweepingsidebands passing through said molecular resonance frequency fromopposite directions in each sweep cycle, transmitting said sweepingsidebands respectively through confined bodies of said gas, demodulatingthe sideband energies passed by the gas to produce pairs of pulses,producing a unidirectional control voltage varying in sense andmagnitude in accordance with variation of the time relation between thepulses of successive pairs thereof, and applying said control voltage tosaid microwave oscillator system in sense to minimize variation ofaforesaid time relation.

10. A system for stabilizing the operating frequency of an oscillatorwith respect to a standard frequency which comprises means formodulating oscillations generated by said oscillator to producecyclically varying sidebands respectively having mean frequencies higherand lower than said standard frequency which in a sweep cycle of'saidsidebands pass through said standard frequency in opposite sense, meansfor deriving control signals as said sweeping sidebands pass throughsaid standard frequency, and means for controlling the frequency of saidoscillator in response to said control signals to maintain a fixed timerelation between successive control signals in predetermined sweepcycles of said varying beat frequency.

11. A two-channel servo-system for stabilizing the frequency of ahigh-frequency oscillator with respect to a standard frequency whichcomprises two high-Q circuit elements both resonant at said standardfrequency and respectively included in said channels, means, includingmodulating means for said high-frequency oscillator and a low-frequencysweep oscillator, for producing sidebands respectively impressed uponsaid elements and sweeping the resonant frequency thereof from oppositedirections in each sweep cycle, means for demodulating the outputs ofsaid high-Q elements to produce pairs of pulses in successive sweepcycles, and a coincidence detector upon which said pairs of pulses areimpressed for derivation therefrom of a frequencycontrol voltage forsaid high-frequency oscillator which varies in sense and magnitude inaccordance with frequency deviations of said oscillator.

12. A system as in claim 11 in which both of the high-Q circuit elementsare resonant at the desired operating frequency of the oscillator forstabilization of the oscillator frequency at the standard frequency.

13. A system as in claim 11 in which the high-Q devices are resonant ata standard frequency olfset from the desired operating frequency of theoscillator and in which the modulating means comprises two modulators,one for producing upper and lower sidebands of the oscillator frequency,and the other for modulating one of said sidebands to produce sidebandfrequencies respectively higher and lower than the offset standardfrequency.

14. A system as in claim 11 in which the modulating means thereofvariably modulates the oscillator for transmission of intelligence andin which the aforesaid upper and lower sideband frequencies arerespectively selected for transmission in the different channels byfrequencyselective devices.

15. A system as in claim 11 in which the modulating means thereofapplies a fixed frequency to produce upper and lower sidebands whichcontinuously differ from said operating frequency of the oscillator by afixed amount.

16. A system as in claim 11 in which an output circuit of thehigh-frequency oscillator includes a mixer common to said channels andin which said modulating means includes a modulator coupled to saidmixer for production of sideband frequencies respectively higher andlower than the standard frequency.

17. A system for automatically stabilizing the' frequency of a microwaveoscillator with respect to a molecular resonant frequency of a gascomprising a coincidence detector for producing a frequency-controlVoltage for said oscillator which varies with variation of the timing ofpulses applied to two input circuits of the detector, each of saiddetector input circuits including a closed chamber containing a body ofsaid gas at low pressure, a demodulator in circuit between said chamberand said detector, a mixer in circuit in advance of said chamber, and amicrowave filter in circuit in advance of said mixer, modulating meansfor said oscillator producing upper and lower sideband frequenciesrespectively selectively transmitted by said microwave filters of therespective input circuits, and means for producing a cyclically varyinglow frequency and impressing it upon said mixers toproduce sweepingsidebands passing through said molecular resonant frequency in oppositedirections in each sweep cycle whereby demodulation of the microwaveenergies transmitted by said bodies of gas provides pulses for saidinput circuits whose relative timing is a measure of the frequencydeviation of said oscillator.

18. A two-channel system for producing an electrical output varying as afunction of the frequency deviations of an oscillation generating systemwith respect to a standard frequency comprising modulating meansassociated with said generating system for producing upper and lowersideband frequencies respectively higher and lower than said standardfrequency, a coincidence detector having two input channels includingfilters respectively selectively transmitting said upper and lowersideband frequencies,

a pair of mixers respectively included in said channels beyond saidfilters for selective impression thereon of said upper and lower`sideband frequencies, a low-frequency generator of cyclically varyingfrequency coupled to said mixers for production of sidebands sweeping inopposite directions through said standard frequency, a pair of high-Qcircuit elements resonant at said standard frequency and respectivelyincluded in said channels for impression thereon of the correspondingsweeping sidebands, and a pair of demodulators in circuit between saidhigh-Q circuit elements and said detector respectively in said inputchannels thereof whereby the output of said detector varies in sense andmagnitude in accordance with frequency deviations of the gen'- eratedoscillations.

19. A system as in claim 18 in which the output of said detectorl isapplied to an electrode of an electronic tube of theoscillation-generating system for rigid control of the frequency of thegenerated oscillations.

20. A two-channel servo-system for stabilizing the frequency of anoscillation-generating system with respect to a standard frequencycomprising two filters respectively included in said channels forselective transmission of frequencies respectively higher and lower thansaid standard frequency, two high-Q elements respectively included insaid channels and sharply resonant at said standard frequency, meansincluding at least one modulator for said oscillator for producing upperand lower sideband frequencies respectively corresponding with thetransmission frequencies of said filters, means including a lowfrequencyoscillator of cyclically varying frequency for beating the outputs ofsaid filters to produce sidebands respectively applied to said high-Qelements to sweep the resonant frequency thereof from oppositedirections in each sweep cycle, a coincidence detector, a pair ofdemodulators for demodulating the outputs of said high- Q elements forimpression upon said coincidence detector, and circuit means forapplying the output of said detector as a frequency control for saidoscillator.

21. A system for automatically stabilizing the operating frequency of anoscillator with respect to a standard frequency comprising a coincidencedetector for producing a frequency-control voltage for said oscillatorwhich varies with variation of the timing of pulses applied to two inputcircuits of said detector, each of said input circuits including ahigh-Q device resonant at said standard frequency, a demodulator betweensaid device and said detector. a mixer in advance of said device, and afrequency-selective device in advance of said mixer, modulating meansassociated with said oscillator for producing upper and lower sidebandfrequencies respectively selectively transmitted by saidfrequency-selective devices of said detector input circuits, and. meansfor producing a cyclically varying beat-frequency and impressing it uponsaid mixers to produce sweeping sidebands passing through said resonantfrequency of said high-Q devices in opposite directions in each sweepcycle whereby said demodulators apply to said detector pairs of pulseswith timing varying with the frequency deviation of said oscillator.

LOWELL E. NORTON.

No references cited.

